1. Field of the Invention
The present invention relates to a method of detecting a silicone coating applied to paper webs and the like to provide an easy-release surface. The method can be employed to measure coat weight and/or uniformity of spread of the coating.
2. Description of the Prior Art
Coatings are often applied to paper and similar substrates to produce coated webs for many different uses. For example, a coating composition of pigment (such as clay) and adhesive (such as starch) can produce a coated paper ideally suited for printing. Coated papers are also used in special applications, such as for release paper where the coating prevents another material from adhering too tightly to the coated paper. A common coating for release purposes is a cured polysiloxane resin, commonly referred to as a "silicone" coating.
In commercial coating processes, the coatings are applied to paper at high speeds where operating variables sometime result in a varying amount of coat weight and in poor coating uniformity. Typical quality control standards require the coating to be reasonably uniform in spread and within a desired coat weight range. Determination of such coating quality is usually not possible by visual observation, because the applied coating does not stand out sufficiently on the paper substrate upon which it is coated.
Prior art methods of coat weight include the use of a beta gauge technique or the coating consumption technique. The beta gauge technique is described in U.S. Pat. Nos. 3,019,336 and 3,130,303. The coating consumption technique is reasonably accurate, but the results obtained only yield an average cost weight over a given period of time with no indication of coat weight variation on the web in either the cross direction or machine direction of the web. Neither the beta gauge technique nor the coating composition technique is capable of determining spread uniformity of the coating.
One technique for detecting the location and spread uniformity of a coating is disclosed in U.S. Pat. No. 3,675,015 to Geib. The Geib patent discloses a method of evaluating the location and uniformity of a normally transparent abherent coating on a substrate by incorporating a fluorescent dye into the coating formulation and then viewing the coated object under ultraviolet light. One advantage of using fluorescent dyes is that the dye is not readily observed with the naked eye, but is observable when illuminated with ultraviolet (UV) light. Therefore, the coating operator can detect the coating with an inexpensive UV apparatus, but purchasers or users of the coated material will not notice the dye.
In a manner similar to that of Geib, U.S. Pat. No. 3,118,060 to Klein and U.S. Pat. No. 3,956,630 to Mellows disclose methods for determining coat weight by application of a fluorescent dye into the coating material and measuring the level of fluorescence in the coating. The level of fluorescence has been found to be reasonably proportional to the coat weight.
The use of fluorescent dyes to determine coating uniformity and coat weight is quite useful, because it is a non-destructive technique that can give immediate readings of the coating quality and because it does not detract from the appearance of the product. Furthermore, it can be used to identify coated paper products after they have been sold. Unfortunately, fluorescent dyes have not been found satisfactory for detecting coatings of cured polysiloxane resins, commonly used for release papers, for one or more of the following reasons. 1. The dyes inhibit curing of the silicone coatings. 2. The dyes do not always fluoresce in silicone coatings. 3. If the dyes do fluoresce in silicone coatings, they often have unacceptably low fluorescence levels. 4. The dyes frequently loose their fluorescence when the silicone coatings are cured. 5. The dyes do not disperse adequately in the silicone coatings.